In a server, a network device, and others, a semiconductor chip (IC chip) and a plurality of communication modules are mounted on a substrate generally called a motherboard. Here, the throughput of the semiconductor chip (IC chip) has been rapidly improved with line thinning of a semiconductor manufacturing process. With the improvement in the throughput of the semiconductor chip, increase in speed of digital signals inputted to and outputted from the semiconductor chip has been advanced year after year. That is, increase in the speed of the digital signals exchanged between the semiconductor chip and the communication module has been advanced year after year. It has been expected that the speed of digital signals inputted to and outputted from a next-generation semiconductor chip and communication module becomes 25 Gbit/sec, and expected that the speed of digital signals inputted to and outputted from a next-next-generation semiconductor chip and communication module becomes 50 Gbit/sec.
However, high-speed digital signals have a large transmission loss in electrical transmission. In other words, high-speed digital signals have severe signal degradation during transmission. For example, in the case of the high-speed digital signals of 25 Gbit/sec a loss of about 0.8 dB/cm occurs on electric wiring formed on a general printed board. Even on electric wiring formed on a sophisticated printed board for high-speed signals, a loss of about 0.4 dB/cm occurs.